The field of the invention relates to magnetic regeneration and more particularly to active magnetic regeneration.
Active magnetic regeneration combines the operation of a regenerator with the magnetocaloric affect. Regenerators have been used in thermodynamic devices for a substantial period of time. However, the materials in known devices have been passive such as lead and stainless steel. Publications directed to reciprocating magnetic refrigerators such as that of G. V. Brown, Journal Applied Physics, 47, 3673 (1976), J. A. Barclay, O. Moze, and L. Paterson, Journal Applied Physics, 50, 5870 (1979) exist, and J. A. Barclay and W. A. Steyert, Los Alamos Scientific Laboratory report LA-8134 (February 1980).
U.S. Pat. No. 4,069,028 to Brown discloses an apparatus and method for magnetic heat pumping. In this device a rare earth material such as gadolinium is used in a fluid regenerator. The temperature and the applied magnetic field of the rare earth are controlled, causing the rare earth to cycle through a temperature-magnetic entropy loop. Brown also discloses that the system may use more than one working substance and suggests the use of gadolinium which may span approximately 20.degree. C. near room temperature in such a loop. Brown also suggests that by providing a regenerator, a room temperature spread between a heat sink and a load may be expanded even further. The Brown Patent references U.S. Pat. Nos. 2,619,603 and 2,589,775 to Chilowsky, the former disclosing that nickel alloys may be used, and that a cascade of several alloys with graduated variations in Curie temperature may be provided to allow larger temperature intervals to be spanned in the refrigeration cycle. The latter suggests employing the heating and cooling effect resulting from passing the material through the Curie point or in the vicinity of the Curie point in order to enhance the desired heating or cooling. The Brown device and method of utilizing it are distinguishable from that of the present invention in several ways. The Brown device uses a fluid generator, separate from its magnetic material, which provides heating or cooling upon magnetization of demagnetization. The present invention utilizes the magnetic material itself as the regenerator as well as a heating or cooling mechanism. The Brown device using a single magnetic material plus a fluid regenerator has a larger temperature span than a magnetic material without a regenerator. However, the temperature span is still limited by the decrease in entropy change as the operating temperature deviates from the Curie temperature of the magnetic material. The maximum practical temperature span of a Brown device is about 60- 80 K, i.e., a span of 30-40 K on either side of the Curie temperature. The only way to increase the temperature span is to stage devices with several heat exchangers to couple stages. In practicing the present invention, a plurality of layers of ferromagnetic materials is disposed in a regenerator such that each material operates near its Curie temperature to provide a large temperature span in a single device, such as 4 K to 300 K or even larger. This is accomplished by the unique combination of refrigeration and regenerative parts and steps. Brown's device provides refrigeration at only one temperature, the coldest temperature. The present invention can provide refrigeration along the entire temperature span of the device because each of the distributed segments of the regenerator executes its own refrigeration cycle. The "distributed refrigeration" feature of the present invention is desirable in many applications such as liquefaction of cryogens and is also advantageous when compared to gas refrigeration cycles where refrigeration is only provided when expansion occurs at several distinct points in a cooling process.